(a) Field of the Invention
The invention described herein relates to pilot lights, and in particular is applicable to pilot lights used for monitoring industrial control circuits.
(b) The Prior Art
A variety of circuit arrangements have been adopted for pilot lights used in the monitoring of control circuits that govern the operation of machines and manufacturing processes. Some pilot lights merely comprise lamps for connection to the circuit elements they are to monitor, so that the lamps light-up when the circuit elements are operated. In these pilot lights the lamp voltages are the same as that of the control circuits in which they are used, which typically may be 110 volts. When it is desired to have a lower voltage lamp in a pilot light a transformer is included within the pilot light housing, with the lamp being connected across the transformer secondary winding. Lamp voltages may then be decreased to a typical value of 6.3 volts. Such a low lamp voltage permits replacement of burned out lamps without shutting down associated equipment. Also, the lamp filament will be thicker than for high voltage lamps, rendering the lamp sturdier and less susceptible to failure in environments where vibration and shock become a factor of lamp life.
In other pilot lights additional circuit components are included in the light assembly to provide a test function, in which the integrity of the lamp may be tested to check whether the lamp is burned out or inoperative. To include such a test function for a pilot light, a test terminal is added to the light assembly that supplements the usual signal terminal (for connection to the device being monitored) and a line terminal (for making a return connection to the control circuit). Internal pilot light connections are also provided that apply a test voltage to the lamp whenever an external voltage is applied to the test terminal. Whenever a test voltage is applied, it is necessary to isolate the test voltage from the signal terminal, so that such test voltage is not fed to the control circuit being monitored. Similarly, the signal voltages applied to the signal terminal should be isolated from the test terminal, so that they are not fed through the test circuit to the other pilot lights.
Isolation between signal and test terminals may be obtained by use of an electromechanical relay, with the relay coil being energized by a voltage applied to the test terminal to switch the movable relay contacts between the signal and test terminals. Then, when a voltage from one of these terminals is applied to the lamp, the other terminal is isolated by an open circuit. Such a use of a relay lends itself to the inclusion within a pilot light of a step-down transformer to achieve a low voltage lamp, and a combination of a relay and transformer has been used extensively.
Isolation may also be obtained by use of solid state circuitry in place of a relay. Blocking diodes connected to the signal and test terminals have commonly been used for this purpose. A diode connected to the signal terminal only allows current flow from the signal terminal toward the lamp, and blocks current flow in the reverse direction. Similarly, a diode connected to the test terminal only allows current flow from the test terminal toward the lamp, and blocks current flow in the reverse direction. However, it has not been practical in commercial pilot lights to include a step-down transformer, to provide a low voltage lamp, while still using only three terminals for the pilot light. This is because the diodes would be in the transformer primary winding circuit, and would transmit pulsating d.c. current to the transformer making it an inefficient device.
Some pilot lights provide a diode type isolation with a step-down transformer for the lamp by including a separate pair of test terminals that apply a separate, reduced test voltage to the pilot light. The pilot light transformer then only handles normal signal voltages, and isolation diodes can be incorporated in the transformer secondary winding circuit, rather than in the primary winding circuit. In this construction there is the disadvantage of introducing a fourth terminal and requiring an extra transformer for supplying the low voltage test current. These disadvantages have reduced commercial acceptance of this construction.
Another suggestion has been the use of a triac in a pilot light that is connected in series with the lamp illuminating circuit. The triac is switched into a conducting state by voltage signals applied either to a signal terminal connected to the device being monitored, or a test terminal connected to a test switch. This approach, however, again requires a four terminal pilot light, and has not received commercial recognition. Instead of such construction it would be desirable to have a solid state pilot light incorporating a low voltage lamp, but with only three external terminals.